Electron tube structure for the production of annular beams of electrons



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' ELECTRON TUBE STRUCTURE FOR THE PRODUCTION OF ANNULAR BEAMS OFELECTRONS Filed Oct. 5, 1946 l l vlils.

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Patented July 28, 1953 ELECTRON TUBE STRUCTURE FOR THE PRODUCTION OFANNULAR BEAMS OF ELECTRONS Leonard Francis Broadway, Ickenham, NormanCharles Barford, Southend, and Albert Frederick Pearce, Hampton Hill,England, assignors to Electric & Musical Industries Limited, Hayes,

England, a company of Great Britain Application October 5, 1946, SerialNo. 701,522 In Great Britain June 25, 1943 Section 1, Public Law 690,August 8, 1946 Patent expires June 25, 1963 11 Claims. 1

This invention relates to the production of hollow beams of electrons.

In electron discharge devices of the velocitymodulation type it issometimes necessary to employ hollow beams of electrons particularlywhere a beam is required for high powered devices. When employingcircular hollow beams in such devices it is desirable that the meandiameter and cross-sectional area of the annulus should be constantalong the length of the beam, but it is found, however, that owing tospace-charge effects the mean diameter of the beam tends to increase asthe beam proceeds from the cathode so that the general shape of the beamis of outwardlydiverging conical form. Similar effects also arise in thecase of hollow beams which have shapes other than a circular form incross-section, such as an elliptical, square or rectangularcross-section. The term annular employed herein is intended to includethese and other hollow shapes.

It is the object of the present invention to compensate for this outwardspread in an annular beam of electrons.

According to one feature of the invention there is provided a circuitarrangement embodying an electron discharge device having means forgenerating an annular beam of electrons and having electrodes arrangedinternally and externally of said generated beam, said electrodes beingmaintained at such potentials as to produce an outwardly directed fieldwhich serves to compensate substantially wholly or in part for theoutward spread in the said beam which would otherwise occur.

According to another feature of the invention there is provided anelectron gun for generating an annular beam of electrons comprising anannular cathode, a cathode shield electrode means and acceleratingelectrode means when in the outer part of said cathode projects in thedirection of the axis of the gun to an extent greater than the innerpart of said cathode, the arrangement being such that if suitableoperating potentials are applied to said gun the shape of said cathodeintroduces an outwardlydirected field which serves to compensatesubstantially wholly or in part for the outward spread in the generatedbeam which would otherwise occur.

According to another feature of the invention there is provided anelectron gun for generating an annular beam of electrons comprising acathode, a cathode shield electrode means comprising a pair ofelectrodes providing an annular space through which the generated beamis arsaid gun said electrodes introduce an outwardlydirected field whichserves to compensate substantially wholly or in part for the outwardspread in the generated beam which would otherwise occur, the cathodebeing shaped, if desired, as above described.

According to yet a further feature of the invention there is provided anelectron gun for generating an annular beam of electrons comprising acathode, a cathode shield electrode means and accelerating electrodemeans comprising a pair of electrodes providing an annular space throughwhich the generated beam is arranged to pass, wherein the electrodes ofsaid pair are asymmetrical with respect to the generatrix of saidannular beam, the arrangement being such that if suitable operatingpotentials are applied to said gun said electrodes introduce anoutwardly-directed field which serves to compensate substantially whollyor in part for the outward spread in the generated beam which wouldotherwise occur, the cathode being shaped, if desired, as describedabove and the cathode shield electrode means being constructed, ifdesired, as described above.

In order that the said invention may be clearly understood and readilycarried into effect it will now be more fully described with referenceto the accompanying drawings, in which:

Figure 1 illustrates diagrammatically in longitudinal cross-section aknown form of electron gun for producing an annular beam of electrons,and

Figures 2, 3, 4, 5, 6, '7 and 8 illustrate similar views of variousembodiments of the invention for the purpose of producing anoutwardly-directed electric field.

Referring first to Figure 1 of the drawings, the electron gun showncomprises a circular annular cathode In which is surrounded by annularcathode shield electrode means H which comprises a pair ofconcentrically arranged circular cylindrical electrodes I2 and I3 andannular accelerating electrode means M which also comprises a pair ofconcentrically arranged circular cylindrical flanged electrodes l5 and[6. These electrodes are symmetrically arranged about the axis H andprovide aligned annular spaces through which the beam passes. Inoperation the cathode Ill may be maintained at zero potential, thecathode shield means ll at a negative potential and the acceleratingmeans I4 at a positive potential. Where the arrangement is intended toproduce an annular beam of electrons in a velocity-modulated electrondischarge device the electrodes I5 and I6 constituting acceleratingelectrode means I4 may form part of the Wall of the hollow resonator 20employed in such a device, as shown in Figure 2, the annular space shownbetween the electrodes I5 and I6 forming an annular aperture throughwhich the electrons pass into the hollow resonator and toward anelectrode 2|. It is found when employing an electrode constructionsimilar to that shown in Figure 1 that spacecharge effects tend toprevent the generation of an annular beam of electrons having a constantcross-sectional area and mean diameter throughout its length since sucheffects tend to cause the beam to spread outwardly between the electrodemeans II and I4 and impart to the electron beam a general shape in theform of an outwardly diverging cone having as its axis the axis a-a. Thedivergenceof the inner surface of the beam towards the axis a-a isnegligible whilst the divergence of the outer surface of the beam awayfrom the axis a-a is substantial. In accordance with the invention, inorder to overcome this spread in the electron beam an outwardly directedelectric field is produced, i. e., a field directed away from the axisa-a. By suitable choice of such a field the mean diameter of the beamcan be maintained more constant between the electrode means II and I4.This outwardly-directed field can be produced by maintaining theelectrode I3 at a more negative potential than the electrode I 2 or bymaintaining the electrode I5 at a more positive potential than theelectrode IS. The maintenance of the electrodes of the shield electrodemeans I I and accelerating electrode means I4 at different potentialsmay be inconvenient in practice and, accordingly, it is preferred toproduce the outwardly-directed field by making the cathode ID, thecathode shield electrode means I I or the accelerating electrode meansI4 asymetrical in the manner hereinafter referred to. Several examplesof guns according to the invention will now be described.

In Figure 2 of the drawings the cathode shield electrode means II ismade asymmetrical with respect to the generatrix of the annular beam bymaking the inner electrode I 2 shorter than the electrode I3 which hasthe effect, if these two electrodes are maintained at the samepotential, of producing an outwardly-directed field to compensate forthe outward spread of the electron beam. Typical dimensions of theoathode and the cathode shield electrode means II may be as follows. Thediameter of the electrode I 2, 33 mms., the inner diameter of theannular cathode, 35 mms., the outer diameter of the annular cathode, 41mms., and the diameter of the electrode I3, 43 mms. The emitting surfaceof the cathode shown in Figure 2 is concave and has a radius ofcurvature of 5 mms., the electrode I3 projects above the peripheraledges of the cathode a distance of 2.5 mms., whilst the electrode I2projects above the peripheral edges of the cathode a distanc of 2 mms.

In Figure 3 of the drawings the cathode shield electrode means I Icomprises a pair of concentric rings II and I8 and the asymmetry withrespect to the generatrix of the annular beam is introduced by disposingthese rings, however, in differout planes with the ring I! nearer to thecathode I0. In Figure 4 the rings l1 and I8 instead of being arranged inplanes are arranged to conform to the surface of a wide-angled cone.

In the example shown in Figure 5 of the draw ings, asymmetry isintroduced in the cathode, the outer part of the cathode i0 projectingin the direction of the axis of the gun to a greater extent than theinner part, as shown. The cathode in this figure is still concavealthough, if desired, the emitting surface of the cathode may be fiat incross-section and may conform to the surface of a wide-angled cone, asshown in Figure 6.

In the example of the invention shown in Figure 7, the acceleratingelectrode means I 4 is asymmetrical with respect to the generatrix ofthe annular beam, the electrode I5 of the accelerating electrode meansbeing disposed closer to the cathode ll! compared with the electrode IB,

as shown, whilst in Figure 8 a similar arrangement is adapted with theexception that the flanges of the electrodes I5 and IS are arranged toconform to the surface of a wide-angled cone, as shown.

Although in Figures 1, 2, 3, 4, '7 and 8 the cathode illustrated has aconcave-emitting surface, it will be understood that this surface may,if desired, be flat or of any other suitable shape.

In some cases it may be desirable to make the mean diameter of theannular aperture in the electrodes I5 and I8 slightly smaller than themean diameter of the annular cathode II It will be appreciated that theinvention is not limited to the production of annular beams of circularform in cross-section, since it is equally applicable to the productionof annular beams of elliptical, square, rectangular or. other hollowshapes. The electrodes of the guns used for generating such beams willbe similar in longitudinal section to the section of the electrodesshown in Figures 2 to 8 but in elevation the shape of the cathode andthe shape of the electrodes I2, I3, I5 and I6 will be such as to conformto the shape of the beam it is desired to generate.

The invention can also be applied to guns designed to produce arectilinear flow of electrons as described in the Journal of AppliedPhysics vol. II August 1940, page 548 and British patent specificationNo. 545,835. Where the invention is applied to the cathode shieldelectrode means of such a gun the asymmetry with respect to thegeneratrix of the annular beam can be introduced by making the angularposition of one electrode of said shield electrode means relative to thegeneratrix of the annular beam different from the other electrode. Wherethe invention is applied to the accelerating electrode means, oneelectrode of said means will be differently shaped compared with theother electrode.

Although in the above examples of the invention various constructionsare shown for producing an outwardly-directed field, it will beunderstood that, if desired, a combination of these constructions may beemployed. Also diiferent potentials may be applied to the individualelectrodes I 2, I3 and I5, I 6 of Figures 2 to 8 for the purpose ofassisting in the production of the outwardly-directed field. Further theinvention is not limited in its application to electron dischargedevices of the velocitymodulation type.

What We claim is:

1. An electron gun for generating an annular beam of electronscomprising an annular cathode, an annular accelerating electrode meansspaced from and axially aligned with said cathode to define an annularbeam path therebetween, and an annular shield electrode means coaxiallydisposed adjacent said cathode, one of said cathode and said shieldelectrode means including inner and outer annular parts located onopposite sides of said annular beam path in the region extending fromsaid accelerating electrode means to and including said cathode, saidparts being asymmetrical with respect to the generatrix of said annularbeam, whereby an outwardly-directed electric field will be establishedacross said annular beam during operation of said electron gun forpreventing spreading of said annular beam.

2. An electron gun according to claim 1, wherein said parts are separateelements having separate terminals for maintaining said inner part at ahigher potential than said outer part to assist said asymmetricalarrangement in establishing said outwardly-directed electric field.

3. An electron discharge device comprising an annular cathode electrodemeans for generating an annular beam of electrons, an annular shieldelectrode means adjacent to said cathode electrode means, and a hollowresonator spaced from said cathode electrode means and having a wallprovided with an annular apertur axially aligned with said annularcathode and shield electrode means, the portions of said wall adjacentthe inner and outer boundaries of saidannular aperture constituting anannular accelerating electrode means for said beam, one of saidelectrode means being asymmetrical with respect to the generatrix ofsaid beam, whereby a radial electric field will be established acrossthe beam path during operation of said device, for preventing outwardspreading of said annular beam.

4. An electron gun for generating an annular beam of electronscomprising an annular cathode and an annular accelerating electrodemeans spaced from and axially aligned with each other to define anannular beam path therebetween, and an annular shield electrode meanscoaxially disposed adjacent said cathode, one of said cathode and saidshield electrode means including inner and outer annular parts locatedon opposite sides of said annular beam path in the region extending fromsaid accelerating electrode means to and including said cathode, saidinner electrode being spaced from said accelerating electrode means agreater distance than said outer part, whereby an outwardly-directedelectric field will be established across said annular beam duringoperation of said electron gun, for preventing spreading of said annularbeam.

5. An electron gun according to claim 4, wherein the outer part of saidcathode projects toward said accelerating electrode means to a greaterextent than its inner part, to establish said outwardly-directedelectric field.

6. An electron gun for generating an annular beam of electronscomprising an annular cathode and an annular accelerating electrodemeans spaced from and axially aligned with each other to define anannular beam path therebetween, and an annular shield electrode meanscoaxially disposed adjacent said cathode, said shield electrode meansincluding inner and outer annular parts located on opposite sides ofsaid annular beam path in the region extending from said acceleratingelectrode means to and including said cathode, said inner part beingspaced from said accelerating electrode means a greater distance thansaid outer part, whereby an outwardlydirected electric field will beestablished across said annular beam during operation of said electrongun, for preventing spreading of said annular beam.

7. An electron gun for generating an annular beam of electronscomprising an annular cathode, an annular accelerating electrode meansspaced from and axially aligned with said cathode, and an annular shieldelectrode means coaxially disposed between said cathode and acceleratingelectrode means and including a first ring of diameter smaller than saidcathode and a second ring of diameter larger than said cathode, saidfirst ring being spaced from said accelerating electrode means a greaterdistance than said second ring, whereby an outwardly-directed electricfield will be established across said annular beam during operation ofsaid electron gun, for preventing spreading of said annular beam.

8. An electron gun for generating an annular beam of electronscomprising an annular cathode and an annular accelerating electrodemeans spaced from and axially aligned with each other to define anannular beam path therebetween, and an annular shield electrode meanscoaxially disposed adjacent to said cathode, each of said electrodemeans including inner and outer annular parts located on opposite sidesof said annular beam path, at least a portion of each of said parts ofsaid shield electrode means being located between said cathode and saidaccelerating electrode means, the spacing between the inner parts ofsaid electrode means being different from the spacing between the outerparts thereof, whereby an outwardly-directed electric field will beestablished across said annular beam during operation of said electrongun, for preventing spreading of said annular beam.

9. An electron gun according to claim 8, wherein said parts of at leastone of said electrode means are separate elements having separateterminals for maintaining said inner part at a higher potential thansaid outer part, to assist said different spacing in establishing saidoutwardlydirected electric field.

10. An electron gun according to claim 8, wherein the spacing betweensaid inner parts is greater than the spacing between said outer parts.

11. An electron gun according to claim 8, wherein the spacing betweensaid inner parts is less than the spacing between said outer parts.

LEONARD FRANCIS BROADWAY. NORMAN CHARLES BARFORD. ALBERT FREDERICKPEARCE.

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